Most conventional propellants, particularly gun propellants, utilize a matrix component such as nitrocellulose plus nitrate esters such as nitroglycerine, and/or nitroguanidine, forming high energy compositions which, unfortunately, can be set off or initiated by sympathetic detonation such as by a neighboring explosion. Because of the potential danger in storing large amounts of propellants of such type, there has been a long standing research effort to reduce propellant sensitivity without significant sacrifice in energy content (e.g. heat of explosion) or loss of desired ballistic characteristics.
One promising approach for developing less sensitive gun propellants has involved the use of high-energy nitramines such as alkylnitrato nitramines as substitutes for sensitive esters such as nitroglycerine in multi-based propellants.
Nitramines of such type, their substitution and preparation, are disclosed, for instance, in U.S. Pat. No. 2,461,582 of Wright et al. and U.S. Pat. No. 2,485,855 of Blomquist et al., in which an ethanol-amine or N-alkyl substituted ethanol-amine plus acetic anhydride are used as reactants.
As noted in Blomquist, however, there is a tendency for such high energy nitramines to migrate and crystallize out of a colloided matrix such as nitrocellulose during storage, resulting in substantial unplanned changes in sensitivity and ballistic properties.
In the present invention this tendency of the nitramines to crystallize out of the matrix is avoided, consequently improving the storage stability of the propellant. The present invention is a high energy nitramine-type plasticizer system suitable for use with nitrocellulose/nitroguanidine-type double base propellant.
This invention also provides a gun propellant composition of low sensitivity containing nitrocellulose/nitroguanidine or other energy adjustment component combined with nitroxyalkyl nitramine plasticizer.
The present invention is a propellant composition, and the method of making it, wherein the composition comprises
A. a matrix component, such as nitrocellulose, PA1 B. an energy adjustment component; and PA1 C. an effective amount of plasticizer component capable of gelation of the matrix component and comprising two nitratoalkyl nitramines:
i. a high energy nitratoalkyl nitramine (i.e. based on heat of explosion) of the formula ##STR1## in which R is defined as --Alk--O--NO.sub.2, H, or a 1-2 carbon monovalent aliphatic group; and Alk is individually defined as a 1-2 carbon divalent aliphatic chain; wherein said high energy alkyl nitramine being at least partly soluble or miscible in a second nitratoalkyl nitramine; and PA2 ii. said second nitratoalkyl nitramine having a lower energy content (i.e. heat of explosion) than the high energy nitramine component of Formula I and represented by the formula ##STR2## in which R' is individually defined as a 2-5 carbon monovalent aliphatic group of different molecular structure from the R group of formula I and n is a positive integer not exceeding about 2.
The ratio of matrix component A/energy component B/plasticizer component C of the propellant composition is about 4-5/1-2/3-4 in parts by weight based on propellant composition, in the cumulative presence of up to about 6% by weight, based on propellant composition, of one or more conventional additives including stabilizers such as ethyl centralite, opacifiers such as carbon black, and flash suppressants such as KNO.sub.3 or K.sub.2 SO.sub.4.
For present purposes the ratio by weight of high energy nitratoalkyl nitramine-to-nitramine of lower energy content in the (C.) component is preferably about 1-5 to 5-1 in parts by weight, and the R and R' substituent groups within formulae I and II are molecularly dissimilar in each plasticizer component.
Of particular interest, for purposes of the present invention, is the use of normally solid high energy nitratoethyl nitramine ingredients in which the definition of R in formula I is nitratoethyl or methyl, and Alk is --CH.sub.2 CH.sub.2 --, while the R' group (formula II) is preferably a 2 to 4 carbon monovalent alkyl group such as an ethyl, propyl, butyl or pentyl substituent. The mixture of plasticizers is adjusted to balance the energy requirements met by the higher energy nitramine with the enhancement of the solubility of the plasticizer provided by the lower energy nitramine. In the absence of the lower energy component, the higher energy component tends to crystalize on the surface of the propellant. This can lead to disadvantages such as variable burning rates, a large difference in physical properties as well as ballistic properties being affected above and below the melting point of the higher energy plasticizer and sensitivity problems caused by the exudation of the higher energy plasticizer in stored propellant that is exposed to prolonged cycling of temperatures in storage.
The term "matrix component" for purposes of the present invention can include one or more polymers selected from the group consisting of nitrocellulose, cellulose acetate, cellulose acetate butyrate, ethyl cellulose, ethyl acrylate-based polymer, and styrene-acrylate type copolymer.
The term "energy adjustment component," for present purposes, comprises generally insoluble energetic solids such as one or more of nitroguanidine, cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX) and ethylene dinitramine (EDNA) and similar recognized components. The term "effective amount", for purposes of the present invention, is defined as about 25%-65% by weight of binder component of the propellant composition (binder not including solids).
Nitratoethyl nitramines of interest for purposes of formula I and II components along with pertinent, physical characteristics are presented in Tables I and II below, in which the energy content of each component is set out as calculated heat of explosion in cal/gm.
TABLE I ______________________________________ ##STR3## (1) Calculated Heat of Physical Melting Explosion Cpd.sup.1 R Form Point (.degree.C.) cal/gm ______________________________________ 1 nitratoethyl solid 52.5 1337 2 methyl solid 38 1113 3 ethyl liquid 5 784 ______________________________________ .sup.1 Assuming use of lower energy formula II component in which it is a least partly soluble or miscible.
TABLE II ______________________________________ ##STR4## (II) Calculated Physical Heat of Form at Melting Explosion Cpd R' Room Temp Point (.degree.C.) cal/gm ______________________________________ 4 ethyl liquid 5 784 5 propyl liquid -2 503 6 butyl liquid -25 259 7 pentyl liquid -30 47 ______________________________________
TABLE III ______________________________________ Cpd# Physical Form Solubility Parameter ______________________________________ 1 Solid 13.1 2 Solid 13.2 3(4) liquid 11.4 5 liquid 11.0 6 liquid 10.6 7 liquid 10.4 ______________________________________
The compounds set forth in Tables I-III are relatively active plasticizers for nitrocellulose and other polymers commonly used as gun propellant ingredients. The relative compatibility (solubility) of a plasticizer in a polymer can be represented by its solubility parameter compared to that of the polymer, i.e., the solubility parameters need to be close together for the plasticizer to be soluble in the polymer. From close inspection of the solubility parameters shown in the table compared to that of nitrocellulose (about 11), the two solid compounds would be less soluble in nitrocellulose than the four liquid ones. If in preparation of a nitrocellulose binder type propellant composition, they were made soluble by a compatibilizing process solvent, then it was found that upon loss of that solvent during drying there was a tendency for exudation and separation. If exudation due to compatibility were to occur, then since the materials are solids, they would tend to remain as solvents on the surface of the propellant at room temperature. In the practice of the present invention, one of the compounds from Table I is dissolved in one of the compounds in Table II to yield a solution that is a liquid at room temperature (70.degree. F.). Preferably, nitratoethyl or methyl nitratoethyl nitramine is dissolved in a nitramine selected from the group consisting of ethyl, propyl, butyl and pentyl nitratoethyl nitramine. Most preferably, the methyl nitratoethyl nitramine is mixed with the ethyl nitratoethyl nitramine.